* Learnt is more common in British English, and learned in American English.
* There are a number of verbs of this type (burn, dream, kneel, lean, leap, spell, spill, spoil etc.).
* They are all irregular verbs, and this is a part of their irregularity.

The *prescriptive* answer is:

“learned” should used in phrases such as “a learned professor”, in which case it is pronounced with two syllables.
“learnt” should be used in phrases like “I learnt a valuable lesson today”.

The *descriptive* answer in British English is:
“learned” is used in phrases such as “a learned professor”, in which case it is pronounced with two syllables.
Either “learnt” or “learned” are used interchangably in phrases like “I learnt a valuable lesson today”.

The *descriptive* answer in American English is:
There is no such word as “learnt”. Use “learned” always.

Learned and learnt are basically the same. They are both used as the past tense form and past participle of “learned”.

1. American speakers mostly use “learned” for the past and the verbal past participle, while “learned” and “learnt” share roughly equal among British speakers.
2. “Learnt” is used as an adjectival past participle in neither American English nor British English.

Linguistic specialists suggest them to be used in the following ways:
LEARN is the present tense, and the conjugated form of the verb “to learn”.
It can be converted to the past tense in the following ways:
I LEARNED lots of things along the way. The main verb is “learned”.
LEARNT is the “past participle” in the following sentences. (Past participles are used in conjunction with other verbs).
I have learnt not to be prejudiced towards Americans or people of other nationalities. (creziauk!)
I had learnt my lesson on peer pressure, the hard way.

To summarize, both learned and learnt are alternative spellings of the past tense and past participle of the verb learn.

Learnt is more common in British English, and learned in American English.